This research is directed toward the further elucidation of the enzymatic mechanisms involved in the assimilatory reduction of nitrate to ammonia. The significance of this pathway is evident from the realization that the various oxidized inorganic forms of nitrogen represent, for all life forms, the ultimate nitrogen source. Nitrate assimilation is a two step process: the 2-electron reduction of nitrate to nitrite, followed by the 6-electron reduction of nitrite ion to ammonia. In Neurospora crassa, the nitrate reductase is a soluble complex catalyzing the sequential transfer of electrons from NADPH, via enzyme- bound FAD, cytochrome b557 and molybdenum, to nitrate. The Neurospora nitrite reductase is also a soluble, flavin-dependent, pyridine nucleotide-specific electron transport system; however, less is known regarding this enzyme and a major goal of this project is its purification and characterization. This enzymatic pathway is induced in the presence of either nitrate or nitrite and repressed by ammonia. The nitrate reductase undergoes a dramatic decrease in activity following maximal induction, and the mechanism which mediates the active disappearance of this enzymatic function in this eukaryotic organism is under investigation. Metabolic regulation of both enzymes via conversions between inactive, reduced forms and oxidized, active forms is also under study.